An aircraft is typically supported by plural pressurized landing gear struts. Each landing gear strut is designed much like, and incorporates many of the features of a typical shock absorber. Designs of landing gear incorporate moving components which absorb the impact force of landing. Moving components of an aircraft landing gear shock absorber are commonly telescopic elements connected by a scissor-link. The scissor-link incorporates a hinge, allowing the arms of the scissor link to move with the telescopic elements of the strut. An alternate type of landing gear incorporates a trailing arm design, where the main supporting element of the landing gear is hinged with a trailing arm. The hinge design of major elements of this gear performs the function of the scissor link. The shock absorber of the landing gear strut comprises internal fluids, both hydraulic fluid and compressed nitrogen gas and function to absorb the vertical descent forces generated when the aircraft lands. Aircraft have limitations regarding the maximum allowable force the aircraft landing gear and other supporting structures of the aircraft can safely absorb when the aircraft lands. Landing force limitations are a key factor in determining the maximum landing weight for a particular aircraft type.
Aircraft landing force is often referred to as Kinetic Energy (KE) and is commonly expressed in the equation—Kinetic Energy equal one half of the mass, times the velocity squared:
  KE  =            mV      2        2  
where:                KE is Kinetic Energy.        m is the mass or weight of the aircraft.        V is the Velocity (vertical speed) at which the mass comes into contact with the ground.        
Aircraft routinely depart from an airport with the aircraft weight less than the maximum take-off weight limitation, but greater than the maximum landing weight limitation. During the flight, in-route fuel is burned to reduce the aircraft weight below the maximum landing weight limitation. Situations can arise where the aircraft has left the departure airport, and the pilot discovers the need to immediately return and land, without the time, nor opportunity, to burn-off the planned in-route fuel. This causes an overweight landing event. When an overweight landing occurs, the FAA (Federal Aviation Administration) and aircraft manufacturer require an inspection of the landing gear and the connections of the landing gear to the aircraft to check for damage. This is a visual inspection done manually, by trained aircraft technicians.
Title 14—Part 25, Chapter § 25.473 of the current FAA regulations define the assumed maximum vertical velocity at which an aircraft would come into contact with the ground as being ten feet per second (10 fps). The origination of this rule comes from the Civil Aeronautics Board—Civil Air Regulations, Part 4, Chapter § 4.24, dated: Nov. 9, 1945. Today an aircraft's maximum landing weight (MLW) limitation is determined by the manufacturer, who must design the structural integrity of the aircraft to allow for the weight/mass to land at an assumed vertical velocity of 10 fps. These limitations assume the aircraft is landing with each of the main landing gear simultaneously touching the ground and the landing force being equally distributed between the two main landing gears. However, cross-wind landings are a common occurrence. In cross-wind situations, the pilot will adjust the lateral angle of the aircraft to lower the wing pointed in the direction of the cross-wind. Lowering this windward wing aides in stabilizing the aircraft against a sudden gust of stronger cross-wind; but also increases the possibility that the aircraft will have an asymmetrical landing gear touch-down. Currently there are no devices installed on aircraft to monitor individual landing gear touch-down velocities.
The present invention will describe an alternate means to perform the required aircraft inspection, by automatically sensing aircraft landing gear strut, touch-down velocities, measured during the initial ground contact of each respective landing gear, at every landing event; and determining if the touch-down forces have exceeded the aircraft limitations.
As the aircraft descends towards the runway, the landing gear is extended. The landing gear maintains a pre-charge pressure within the shock strut, even though no weight is applied to the strut. The pre-charge pressure is a relatively low pressure, which is maintained to insure the landing gear shock absorber component is extended to full strut extension, prior to landing. At full extension, the shock absorber can absorb its maximum amount of landing force. As the aircraft landing gear come into initial contact with the ground, the minimal pre-charge pressure within the strut easily allows for the shock absorber to begin compressing. As the strut continues to compress, internal strut pressures increase, allowing the strut to absorb the landing force.
Multiplying the strut pre-charge pressure times the cross-sectional area of the shock strut will determine a weight value which corresponds to the amount of opposing force that would reduce the rotation of the scissor-link or trailing arm hinge of the landing gear during the initial contact with the ground. Any opposing force applied to the initial touch-down velocity would reduce that velocity by reducing the rate of landing gear scissor link hinge rotation.
Subsequent adjustments to this reduction in velocity can be made to correct for this opposing force, in the determination of actual Kinetic Energy transferred during the landing event. As the landing gear comes into initial contact with the ground, the strut begins to compress, thereby increasing the pressure within the shock absorber. Increases in pressure, beyond the pre-charge pressure, creates additional resistance to the compression of the landing gear strut.
Beyond the measurement of initial touch-down velocity, the rate of the slower rotation associated with the amount and rate of internal shock absorber pressure is valuable data and will be used in monitoring the landing loads applied to these structural members. Landing load data (corrected from the distortions caused by strut pre-charge pressure and landing gear strut seal friction) accumulated with every aircraft landing event, will be stored and used to build an accurate life history of the landing gear. A comparison is made between actual landing load data and the aircraft manufacturer's assumption of landing gear use or possible abuse; to develop the documentation necessary, with engineering review, to allow increases in the life limitation of the aircraft landing gear system.
The FAA requires flight data recorders (FDR) on transport category aircraft. The FDR incorporates multi-axis accelerometers (located at the center of gravity of the aircraft hull) which measure various shock loads that become evident in an abrupt landing event. The accelerometer data is usually not available unless an accident has occurred, and the FDR is removed from the aircraft, the data downloaded, and then analyzed. Assuming one might attempt to determine aircraft landing gear touch-down velocity from the FDR data, the information would be merely calculations from measurements taken not at the respective landing gear locations of the aircraft, but along the center-line of the aircraft. The FDR calculations would not be associated with the touch-down velocity of any respective landing gear strut, but the velocity of the aircraft hull as a whole.
A research of prior art identifies numerous system which measure aircraft descent velocity. Though it is advantageous for pilots to know the average descent velocity or sink-rate of the aircraft while approaching a runway for landing, the actual descent velocity can vary drastically due to non-pilot actions including such factors as varying wind conditions. The descent velocity of the aircraft hull the does not necessarily indicate the touch-down velocity of any respective landing gear strut as it comes into initial contact with the ground.
Prior art to determine aircraft descent velocity is well documented. Reference is made to U.S. Pat. No. 3,712,1228—Harris; U.S. Pat. No. 6,012,001—Scully, and U.S. Pat. No. 4,979,154—Brodeur. These and other patents describing similar but subtly different techniques teaching the use of various range-finder devices, attached to the aircraft hull, which measure the distance between the aircraft hull and the ground, as well as the rate of change of those measurements. Unfortunately the range-finder devices do not measure the initial touch-down velocity of each respective landing gear, as they come into contact with the ground. For example, as an aircraft approaches a runway for landing, if the pilot properly flares the aircraft, the descent velocity of the aircraft will dramatically reduce just a few feet above the runway surface. During the aircraft flare procedure, a cushion of air is developed by the downward force of air generated by the wing coming near the ground surface. This cushion of air is often referred to as “ground effect” and can substantially reduce the descent velocity of the aircraft. In ground effect, the aircraft is reaching a stall situation which reduces the lifting force generated by the wings. Aircraft wing oscillation can occur, where the aircraft wings flutter from side to side. This is another situation where an asymmetrical landing gear touch-down will occur. Aircraft descent velocity, measured along the centerline of the aircraft, will not detect wing oscillation and will not determine the initial touch-down velocity experienced by each individual land gear, when the aircraft comes into initial contact with the ground.
Additional search of prior art relating to landing gear identified U.S. Pat. No. 2,587,628—King, which teaches an apparatus for testing “yieldable load carrying structures” such as aircraft landing gear. King teaches monitoring the rate of deceleration of the mass supported by the landing gear and the effects on other connected landing gear elements. King teaches the relationship between the telescopic rate of compression of the landing gear, as compared to shear deflection to other structural members of that same landing gear. King teaches apparatus used as a tool to determine the effective change in the fatigue life limitations of a particular landing gear structural component, by tracking the rate of change in force applied to the shock absorbing components attached to said fatigue life limited structural component.
U.S. Pat. No. 3,517,550—Leventhal, teaches the relationship of comparing internal strut pressure increases, as related to the rate of landing gear strut compression, thereby determining the rate of change in descent velocity. Though the approach may appear valid, it is subject to error by its inability to verify, at any given landing event, the exact proportion of gas volume in relation to hydraulic oil volume, within the landing gear strut. My U.S. Pat. No. 6,128,951—Nance teaches the measuring of strut pressure within each landing gear strut, as well as determining the current proportion or ratio of gas to hydraulic oil within each respective landing gear strut. Internal strut pressure, compared to strut extension, is not a linear relationship. Commonly aircraft maintenance technicians observed landing gear struts which appear near deflated, due to hydraulic oil having escaped through the strut seals. Mistakenly assuming the landing gear has lost nitrogen gas, the technician adds additional gas to the strut, thus the landing gear strut is now over-charged with gas. The now changed and unknown volume of gas being compressible and that variance in volume of gas as compared to the unknown volume of non-compressible hydraulic oil having changed, would thereby vary the compression rate of the landing gear strut and generate errors in the velocity calculation. Also, pressure within a landing gear strut is contained by the friction of the landing gear strut seals. My U.S. Pat. No. 5,214,586—Nance teaches distortions in landing gear strut pressure measurements caused by landing gear strut seal friction. Landing gear strut seal friction can distort internal strut pressure measurements by as much as 5% of the applied weight. Attempts to determine initial touch-down velocity of the landing gear strut would be subject to errors caused by the friction of the strut seals distorting pressure measurements and delaying any increases in internal landing gear strut pressure. These delays in any increase in strut pressure due to strut seal friction would distort the accuracy of a direct comparison of rate of internal pressure increases to strut compression.
This invention relates to differences and improvements to the stated prior art. Combining the prior art of Leventhal and Nance might develop a tool to calculate various factors that might distort initial pressure changes as they relate to initial strut compression. An actual vertical velocity measurement, as described herein, would be superior to any calculation using assumed factors. The new invention described herein surpasses the prior art calculations by mechanically measuring the rotation rate of vertically rotating landing gear structural components such as the scissor link hinge and/or trailing arm hinge as the aircraft landing gear strut comes into initial contact with the ground. The arms of the scissor link hinge form an angle. That angle changes as the landing gear comes into initial contact with the ground and begins to compress (as with telescopic type design) or collapse (as with trailing arm design). Measurement of the angle change, as well as rate of angle change, can be accomplished by installing a mechanical rotation sensor at the vertex point of the angle. The rotation sensor is attached to the hinge point of the arms of the scissor link component of the landing gear strut. Measurements of the rotation of the scissor-link hinge can be monitored at rates up to 40,000 samples per second, with the ability of measuring vertex angle changes within 1/100th of a degree. The speed and ability to identify the initial and most minor changes in hinge angle, by hinge rotation, allows the initial touch-down velocity to be measured and determined, before the strut has compressed to a point the touch-down velocity would be reduced by pressure build-up, beyond the pre-charge pressure. Additional measurements, taken after initial rotation, are considered inaccurate due to velocity reduction caused by additional opposing pressure build-up in the strut.